FSI Analysis in Nuclear Power Plant Assessments
The use of finite element codes and CFD codes for the separate analyses of structural and fluid power plant
components is of course well established. However, it can be of great importance to perform also FSI analyses,
in which the fully coupled interaction between structural and fluid components is taken into account. A decoupled
analysis can lead to conservative but also to significant nonconservative results.
We present here briefly some results obtained in the FSI analysis of a Forsmark nuclear power plant.
These results have been furnished to us by Forsmarks Kraftgrupp AB, Sweden.
We consider the vessel shown in the figures. The movie above shows the inside of the finite element model.
An important question to be addressed by the analyst is whether the fluid should be modeled as a full
NavierStokes fluid, whether a nonlinear inviscid fluid assumption is sufficient, or whether the fluid can
be assumed to be an acoustic fluid (inviscid and small deformations). To answer these questions, ADINA
can be used directly with these various assumptions for the fluid, and hence the effect of the assumptions on the
predicted response can be assessed.
For the analysis case considered here, it has been found that assuming the fluid to be an acoustic fluid is
adequate, but the fully coupled fluid structure interactions must be modeled. The figure below
shows a comparison of typical results obtained in another analysis with the corresponding experimental data.
Comparison of results using FSI vs. without FSI, against measured results
The FSI analysis of the vessel when subjected to a pipe break was performed using
 Implicit dynamic analysis, time step size 10^{5} seconds, step size chosen to accurately
integrate the highest frequency of interest in the structure
 15000 time steps
 Linear analysis. Linear structural elements, linear potentialbased fluid elements
(½ρv^{2} effect is neglected)
 Solution time: 3 hours on Linux x86_64 computer (2.66 GHz processor), one processor used. 1 GB memory required
Model used
Mesh used, with about 65000 nodes, consisting of shells, beams and fluid elements
The movie below gives the dynamic displacement magnitude and predicted dynamic mean stress intensity in the core shroud as
a function of time.
The effectiveness of ADINA is evident. Firstly, various complex to simplifying assumptions for the structure
and for the fluid can be made, and the effect of these assumptions can be studied. Secondly, each of the required
analyses can be performed in an efficient manner.
Core shroud — dynamic stress intensity P_{m}
For the dynamic analysis pursued here, the final fully coupled FSI model could be solved on a PC within only 3 hours
computing time. With such low computing time, the model can be used to efficiently make parametric studies, considering,
for example, various loading conditions and boundary conditions.
